Standard & Poor's is saying investors should sell Google's stock because it believes the search leader's decision to buy Motorola Mobility increases the risk to the company and its shares.

Google Inc. said Monday it will pay $12.5 billion for Motorola Mobility, a major maker of phones using Google's Android mobile software. The deal includes mobile patents that could help Google defend itself against rivals.

S&P said Tuesday that while the acquisition would include a patent trove, that might not be enough to keep Google's Android mobile operating software from encountering intellectual-property issues. It downgraded its rating on Google's shares to "Sell" from "Buy."

Further, the ratings service says the transaction will hurt Google's growth, margins and balance sheet.

S&P cut its price target for Google's stock by $200 to $500.

Google shares fell along with the overall market Tuesday, slipping $18.23, or 3.3 percent, to finish trading at $539.